[IanCal]

Although on the flip side, the number also assumes you have no flexibility and don't change your spending according to how much you have left.

It's a tricky one to work out, and 4% is (as you say) definitely not a number you can easily use for retiring in your 30s.

[Mvandenbergh]

Definitely. If you're willing to constrain consumption when required and work again if you have to, things change. Especially if you continue to work just enough in your professional field during early retirement that you can go back to earning an appreciable fraction of your previous earnings on a temporary basis if/when required.